European Patent Publication EP 1 208 766 A2 discloses a like ion generating unit that includes a needle electrode and a ground electrode for developing the corona discharge to generate the ions in the air around the pointed tip of the needle electrode by applying a high voltage across the electrodes. The unit has a dielectric tube which surrounds the needle electrode and the ground electrode in the form of arc or partially cut ring. The needle electrode is held in a center of the dielectric tube, while the ground electrode is disposed at one longitudinal end of the tube in a longitudinally spaced relation to the pointed tip of the needle electrode. However, the use of the tube necessitates a dead space at a portion of the tube away from the ground electrode. For making the unit compact, it could be theoretically possible to make the wall of the tube closer to the needle electrode. However, due to the structural limitation that the needle electrode and the ground electrode are supposed to be mounted in any manner commonly on the inner face of the tube, the above approach might bring about a problem of shortening a creepage distance between the needle electrode and the ground electrode along the common inner face of the tube. When the creepage distance becomes greater than an air path between the pointed tip of the needle electrode and the ground electrode, a harmful arc discharge instead of the corona discharge would certainly develop therebetween the electrodes, failing to generate the ions. Thus, the above approach alone is found unsuccessful to make the whole unit more compact.